Designer ionic liquid crystals based on congruently shaped guanidinium sulfonates

Area of Science:

• Materials Science
• Supramolecular Chemistry
• Organic Chemistry

Background:

• Ionic liquid crystals (ILCs) typically feature rod-like cations and spherical anions.
• Understanding the influence of molecular shape on ILCs is crucial for designing new materials.
• Existing synthesis methods often rely on dissimilar ion shapes.

Purpose of the Study:

• To develop a new synthetic method for ionic liquid crystals using ions of the same molecular shape.
• To investigate the impact of congruent anion shape on mesophase behavior compared to spherical anions.
• To synthesize and characterize novel 4-alkoxyphenylpentamethylguanidinium 4-alkoxyphenylsulfonate and 4-alkoxyphenylpentamethylguanidinium iodide ion pairs.

Main Methods:

• Synthesis of guanidinium-based ionic salts.
• Differential scanning calorimetry (DSC) for thermal analysis.
• Polarizing optical microscopy (POM) for mesophase identification.
• X-ray diffraction (XRD) for structural elucidation.

Main Results:

• All synthesized ionic salts exhibited smectic A mesophases with interdigitated bilayer structures.
• Guanidinium sulfonate ion pairs displayed mesomorphism at shorter alkyl chain lengths (≥C(9)) and lower melting points.
• Guanidinium iodides required longer alkyl chains (≥C(14)) for liquid crystallinity.
• Mesophase range decreased for sulfonate ion pairs with longer chains (≥C(18)), unlike the iodide salts.

Conclusions:

• The use of congruent ion shapes offers a novel approach to synthesizing ionic liquid crystals.
• Anion shape significantly influences the onset and range of mesophase behavior in ILCs.
• These findings provide insights into structure-property relationships for designing functional ionic liquid crystals.